A method is known from German Patent Application No. 195 43 813. It is used in a motor vehicle to determine the positions of radar objects, for example of vehicles driving ahead, using a static multibeam radar, so that then, within the framework of an adaptive cruise control (ACC), the velocity of one's own vehicle is adapted to the velocity of a vehicle driving ahead, and the distance to the vehicle driving ahead can be regulated to a suitable value. The positions of the radar objects are indicated in polar coordinates, thus by distances and directional angles. The distances can be determined on the basis of the signal propagation times of the radar echoes. In addition, using the Doppler effect, the relative velocities of the radar objects can be determined. However, for an error-free distance control (vehicle-to-vehicle ranging), the directional (course) angles of the radar objects are needed in order for a decision to be made as to whether a located (tracked) radar object is a vehicle driving ahead in one's own lane, or a vehicle driving in an adjacent lane that is irrelevant for the distance control.
In the context of a static multibeam radar, the optical axis of the radar system is fixed in relation to the vehicle. It is preferably parallel to the longitudinal axis of the vehicle. This optical axis then expediently forms the reference axis for determining the directional angles. The multibeam radar system has a plurality of receiving elements, each of whose sensitivity maxima are in different receiving directions, so that altogether, therefore, they cover a specific angular range. Since the sensitivy ranges of the receiving elements overlap one another, from one single radar object, one receives radar echoes in a plurality of channels, i.e., in a plurality of receiving elements. For an idealized, nearly punctiform radar object, at a given directional angle, a characteristic phase and amplitude relation exists among the signals received in the various channels. Due to the differences in the propagation time (delay differences) of the radar echoes from the radar object to the various receiving elements, a phase difference is derived which is proportional to the directional angle and to the distance of the receiving elements in the right-angled direction to the optical axis, and is inversely proportional to the wavelength of the radar waves. The amplitude ratios among the received signals are dependent upon the directional angle and upon the sensitivity curves of the receiving elements. They are able to be experimentally determined in advance for the directional angles of interest and recorded in a reference antenna diagram. In this way, by evaluating the phase relations or by evaluating the amplitude relations, or also by combining both evaluation processes (evaluating the complex amplitudes), it is possible to determine the directional angle of a located radar object.
The high-frequency signals received in the various channel are able to be evaluated in a mixing process using a reference frequency, while maintaining the phase and amplitude relations, and converted into low-frequency signals which are able to be evaluated in an evaluation electronics. For example, the low-frequency signals can be digitized using analog/digital converters and then digitally further processed. A frequency spectrum is first recorded for each beam of the multibeam radar, i.e., for each of the low-frequency signals received from the various receiving elements. Each radar object emerges in the spectrum in the form of a peak, whose position is dependent upon the Doppler shift and, thus, upon the relative velocity of the object. When the transmitting frequency of the radar system is modulated, for example when working with a FMCW radar (frequency modulated continuous wave), the position of the peak is also dependent upon the propagation delay. When the transmitted signal is alternately modulated with ascending and descending ramps (ramp waves), the relative velocity of the object can be calculated from the frequency spacing of the peaks obtained at the various ramps, and the distance of the object can be calculated from the average value of the peak frequencies. Any ambiguities in the received signals that arise when simultaneously locating (finding the position of) a plurality of objects, are able to be overcome by varying the ramp slopes in the frequency modulation. Peak pairs which belong together can be identified by the correspondence of the relative velocities and object distances obtained at various ramp slopes.
Since the signals received from the same object in the plurality of receiving elements of the multibeam radar have identical Doppler shifts and also at least nearly identical signal propagation times, the peaks in all the channels are more or less at the same frequency.
In the known method, as a measuring frequency for the angular determination, that frequency is selected which corresponds to the apex of the peak.
However, real (tangible) radar objects, in particular large objects such as trucks, usually have a plurality of centers of reflection, whose radar echoes are superposed in the various receiving elements and interfere with one another. This can degrade the accuracy and reliability of the angular determination.